When prints are produced through printing by an inkjet printer using an inkjet ink composition comprising carbon black as a colorant, the printer nozzle is often clogged arising from poor dispersion of carbon black in the inkjet composition, thereby resulting in poor print quality. Various attempts have been made to prepare aggregation-free carbon black inkjet ink compositions. Some examples of aqueous-based carbon black inkjet ink compositions include those disclosed in U.S. Pat. Nos. 5,571,311 A, 4,530,961, 5,085,698, U.S. Pat. No. 20040123773 A1, U.S. Pat. No. 6,328,393 B1, European Pat. No. EP1887058 A1, and European Pat. No. EP2290021 A1.
For solvent-based inkjet ink system, U.S. Pat. Appl. No. 2013/0030099A1 has disclosed a carbon black ink composition containing an alkane nitrile and a solvent selected from the group consisting of methyl ethyl ketone and cyclohexanone to prevent carbon black from forming intact structure. As a result, a good dispersion could be obtained.
U.S. Pat. No. 8,586,656 B2 has disclosed a particular inkjet ink composition, which provides a good dispersion stability of the carbon black. It comprises a combination of solvent mixture, a pigment, a binder resin, a pigment dispersant, and a solvent, wherein in the solvent is formed of (1) at least one glycol ether and at least one of a lactone compound and 2-pyrrolidone, or (2) at least one glycol ether acetate and at least one of cyclohexane and isophorone. However, the use of large amounts of non-polar solvents leads to low ink conductivity due to their low dielectric constant, thereby limiting their applicability in a continuous inkjet printer.
All of the inkjet ink compositions disclosed by the above patents mainly studied the effect of solvent combinations on dispersion of carbon black in solvents to enhance its dispersion stability. Despite of the success of their approaches, carbon black-based inks often suffer from poor printing reliability in a continuous inkjet printer because the electrically conducting carbon black easily forms an insoluble conductive film on high voltage deflection plates in the printer in the presence of electric field. The insoluble film increases its thickness over time, forming compact film, and finally leading to printing failure. As a result, a special cleaning solution is required to wash the insoluble film.
To prevent insoluble films formed on the print head of the continuous inkjet printer, U.S. Pat. Appl. No. 2013/0027478 has disclosed an ink composition comprising an organic solvent, a binder, and a colorant which is selected from at least two colors of pigments with or without carbon black to produce black colored inkjet ink. It is claimed that reducing or having no carbon black in the formulation can significantly improve the printing stability. However, there is a major concern about color separation because each color of pigment may have different dispersion stability in a particular solvent system.
Some conventional pigment ink compositions comprise a black colored iron oxide pigment particles, a binder, a solvent or solvent mixture, such as those disclosed in European Pat. No. 1840178 A1, U.S. Pat. No. 8,303,838 B2, Canadian Pat. No. 2,770,959 A1, U.S. Pat. No. 8,597,420 B2. However, all of such conventional ink compositions require high energy and time-consuming grinding process to disperse the iron oxide pigment in the solvent. Furthermore, serious sedimentation of the iron oxide pigment in ink formulation often occurs due to its high density.
All of the conventional ink compositions have never used a bottom up process for making surface-modified iron oxide or iron hydroxide nanoparticle as a colorant. In particular, no prior art documents have disclosed ink compositions that contain a superparamagnetic iron oxide or/and iron hydroxide nanoparticle as a colorant to provide excellent printing stability. Furthermore, no prior art documents have disclosed the use of magnetite nanoparticles as an alternative to carbon black in making ink compositions. In addition, no prior art documents have disclosed tuning ink color to, for example, yellow, red, brown and black by varying the crystal structures of the iron oxide or/and iron hydroxide crystals.
Therefore, there is a need for overcoming the disadvantages and problems mentioned above. The present invention overcomes the problems associated with prior arts as mentioned above.